In recent years, a Giant Magnetoresistance (GMR) effect element comprising a multilayer film of a ferromagnetic layer/non-magnetic metal layer and a ferromagnetic tunnel junction (MTJ) comprising a ferromagnetic layer/insulator layer/ferromagnetic layer have attracted attention as new magnetic field sensor and non-volatile magnetic random access memory (MRAM) cell. With respect to the GMR, there have been known a CIP-GMR of a type such that an electric current flows within the plane of the film and a CPP-GMR of a type such that an electric current flows in the direction perpendicular to the plane of the film. The principle of the GMR resides mainly in spin-dependent scattering at the interface between the magnetic layer and the non-magnetic layer, but spin-dependent scattering in the magnetic material (bulk scattering) also contributes to the principle.
For this reason, generally, the CPP-GMR to which the bulk scattering is expected to contribute is larger than the CIP-GMR. There is used a GMR element of a spin valve type in which an antiferromagnetic layer is disposed in the vicinity of one of the ferromagnetic layers to fix the spin of the ferromagnetic layer.
On the other hand, in the MTJ, by controlling the magnetization configurations of two ferromagnetic layers to be parallel or antiparallel to each other using an external magnetic field, a so-called tunnel magnetoresistance (TMR) effect in which the tunnel currents in the direction perpendicular to the planes of the films are different from each other can be obtained at room temperature. The TMR ratio in the tunnel junction depends on a spin polarization P at the interface between the ferromagnetic material and insulator used, and it is known that when spin polarizations of two ferromagnetic materials are respectively taken as P1 and P2, a TMR ratio is generally given by the following formula (1).TMR=2P1P2/(1−P1P2)  (1)A ferromagnetic material has a spin polarization P which satisfies the relationship: 0<P≦1. Conventionally, an Al oxide film (AlOx) having an amorphous structure and a (001) plane-oriented MgO film have been used as a barrier. The former is prepared by forming a film of Al metal by a sputtering method or the like and then oxidizing the resultant film by a plasma oxidation method or the like, and it has been well known that the Al oxide film has an amorphous structure (non-patent document 1). On the other hand, as a method for preparing the MgO barrier, a method is used in which a MgO target is directly sputtered or a MgO shot is evaporated using an electron beam.
As can be seen from the formula (1), when using a ferromagnetic material having a spin polarization P=1, an infinitely large TMR is expected. A magnetic material having P=1 is called a half-metal, and from the band calculations already made, oxides, such as Fe3O4, CrO2, (La—Sr)MnO3, Th2MnO7, and Sr2FeMoO6, half Heusler alloys, such as NiMnSb, full Heusler alloys having an L21 structure, such as Co2MnGe, Co2MnSi, and Co2CrAl, and the like are known as half-metals.
The MTJ has currently been practically used in a magnetic read head for hard disk and a non-volatile magnetic random access memory MRAM. In the MRAM, MTJ are arranged in a matrix form and an electric current is allowed to flow a separately provided wiring to apply a magnetic field to the MTJ, so that two magnetic layers constituting each MTJ are controlled to be parallel or antiparallel to one another, thus recording data of 1 or 0. The recorded data is read utilizing a TMR effect. In such a field of application that needs high-speed operation, an MTJ having a small resistance is demanded. In addition, recently, magnetization switching of an MTJ by injection of a spin polarization current, i.e., so-called spin-transfer magnetization switching is important, and an MTJ having a small resistance is needed therefor. Further, a technique for spin injection into a semiconductor through a barrier is increasingly important in the fields of spin MOSFET and spin transistor. Also in these fields, a barrier having a small resistance is required for gaining a larger on-current.
In this situation, the conventional AlOx amorphous barrier has disadvantages in that the junction resistance is too high, that the interface roughness between the ferromagnetic layer and the barrier layer is marked such that the properties become remarkably uneven, and that the TMR is generally small, and therefore the AlOx amorphous barrier is not suitable for the above-mentioned spintronics device. On the other hand, with respect to the epitaxial tunnel junction using a crystalline MgO barrier, it has been known that the tunnel junction has a large tunnel transmission of a Δ1 band electron with respect to a ferromagnetic layer material having a bcc crystalline structure, such as Fe or FeCo alloys, due to the feature of the electronic structure of the MgO, and therefore has a small tunnel resistance, and further the TMR is largely enhanced due to the coherent tunnel effect (non-patent document 2).
A Co-based full Heusler alloy is an intermetallic compound having a Co2YZ type composition, and it is known that the Co-based full Heusler alloy having an L21 structure or a B2 structure is generally a half-metal. In such a compound, for obtaining an ordered structure, heat treatment is required, and for obtaining a B2 structure, it is generally necessary to heat the substrate at 300° C. or higher or to form a film at room temperature and then subject the resultant film to heat treatment at a temperature of 400° C. or higher. For obtaining an L21 structure, heat treatment at a temperature higher than that required for obtaining the B2 structure is needed. Conventionally, an MTJ using a Co-based full Heusler alloy is prepared using Cr or MgO as a buffer layer on a MgO (001) single crystal substrate and using MgO or amorphous AlOx as a barrier. A MgO barrier is epitaxial-grown on a Co-based Heusler alloy film, and a Co-based full Heusler alloy is also epitaxial-grown on the MgO barrier, so that a B2 or L21 structure can be relatively easily obtained.
The inventors of this application have proposed a Co2FeAlxSi1-x (0<x<1) half-metal Heusler alloy having a controlled Fermi level (patent document 1), and have reported a large TMR at room temperature (non-patent document 3).
It has been pointed out theoretically that the coherent tunnel effect is effective in the Co-based full Heusler alloy (non-patent document 4). However, when the Co-based full Heusler alloy is used as a material for the ferromagnetic layer, the lattice mismatch between the alloy and MgO is large such that many defects, such as dislocation, are caused in the MgO barrier, making it difficult to obtain a high-quality tunnel junction. Particularly, the structure of the Co-based full Heusler alloy on the MgO barrier is likely to be a disordered structure, so that a giant TMR expected from a half-metal is not observed. Further, the momentum in the direction perpendicular to the plane of the film is not conserved due to the formation of disordered structure at the interface, and the enhancement of TMR by the coherent tunnel effect pointed out by the theory is not always observed. On the other hand, the AlOx barrier having an amorphous structure has a problem in that the Co-based full Heusler alloy formed on the AlOx barrier is unlikely to have a B2 or L21 structure, and generally has an A2 structure and hence loses properties of half-metal, making it difficult to obtain a large TMR. In addition, a difficulty of reducing the large junction resistance must be overcome. Further, there is also a problem in that the interface between the Co-based Heusler alloy layer and the AlOx barrier is oxidized when the AlOx barrier is formed.
Recording or reading data using an MTJ needs the application of a bias voltage of several hundred mV to about 1 V. However, the MTJ having an amorphous AlOx barrier or a MgO barrier has a problem in that generally the application of a bias voltage of about 500 mV reduces the TMR value to half of the value at zero bias voltage. The large dependence of the TMR value on the bias voltage is caused mainly by the lattice defect or interface roughness between the ferromagnetic layer and the barrier layer, and therefore in the MTJ having a conventional amorphous AlOx barrier or MgO having large lattice misfit, it is extremely difficult to improve the TMR dependence on the bias voltage.